Obtain an analysis of the biological behavior of the mammary gland, in particular, the possibility of its early neoplastic transformation, when hormonal contraceptives, estrogen and progestin, are present at unphysiological levels during the neonatal period. Effects on other endocrine target tissue should be a part of the project. Approximately 5% of nursing mothers can conceive at six weeks post-partum. Therefore, oral contraception is often started within four to five weeks after parturition. After milk production has been initiated, milk yield is not manifestly impaired by contraceptives. Although it may be accidental, gynecomastia has been described in infants nursed by mothers taking hormone contraceptives. Neonatal exposure of mice, however, to estrogen, androgens, or progestin, increased mammary tumor risk by 12 months of age and progesterone given with estradiol did not modify the pattern of oncogenesis.